Volume 166, September 2021, 108508

Experimental characterization of heat transfer enhancement in a circular tube fitted with Blade inline mixer.

Ramin Zarei, Kiyanoosh Razzaghi, Farhad Shahraki

https://doi.org/10.1016/j.cep.2021.108508 Get rights and content

Highlights

Heat transfer enhancement characteristics in Blade-type elements and inline mixers have been investigated.

The flow swirling in the mixer is mainly due to the centrifugal force induced by the angle between the blades.

Heat transfer in the tube with inserts can be augmented up to 78% relative to the plain tube.

The 120°-blades mixer provides thermal performance factors up to 17% greater than that of a 90°-blades mixer.

Abstract

This paper addresses the experimental study of heat transfer enhancement of laminar flow in a tube fitted with a Blade inline mixer as the swirl-flow generator and under constant surface heat flux. Experiments were carried out with air as heat transfer fluid and two different types of mixer having blades crossing angles of 90° and 120° The flow swirling in the mixer is mainly due to the centrifugal force induced by the angle between the blades, which is responsible for heat transfer augmentation.

The results show that the mixer with 90° blades supplies Nusselt numbers higher than the 120° mixer but, likewise, results in a higher pressure drop. The rate of heat transfer in the tube equipped with inserts was augmented up to 78% and 47% for 90°-blades and 120°-blades mixers, respectively. The enhancement in heat transfer was correlated with the Graetz number that supports the experimental data for both mixers by a deviation within ±10%.

Furthermore, the tube fitted with a 120°-blades mixer yields higher thermal performance with a maximum value of 1.77. Also, the 120°-blades mixer provides thermal performance factors up to 17% greater than that of a 90°-blades mixer for the same pumping power, which implies that heat transfer effects significantly dominate pressure drop effects.

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Introduction

Efficient heat transfer is imperative in many processing industries utilizing heat exchangers. The major tasks mainly focus on size reduction, the approach temperature difference, and upgrading the capacity [1]. Heat transfer enhancement can be performed either actively by the aim of external forces or passively without any external force. Compound enhancement techniques that use two or more passive and/or active methods can also be used.

The effectiveness of each method strongly depends on how heat is transferred as well as how heat exchangers are utilized in processes [2]. The use of swirl-flow devices is one of the most significant passive techniques that are widely used to produce swirl or secondary recirculation in the flow direction. These devices have numerously been investigated and reported in the literature. A good review of heat transfer enhancement techniques by swirl-flow generators can be found in Sheikholeslami et al. [3]. Moreover, a recent review of heat transfer enhancement techniques for single-phase heat exchangers has been presented by Alam and Kim [4].

Several investigations have been focused on heat transfer enhancement by twisted tape and modifications to improve its performance [5], [6], [7], [8], [9]. Manglik and Bergles [5,6] studied swirl flow regimes in a tube with twisted tape under uniform wall temperature. Four regions have been proposed concerning the features of the dominant flow and the mechanisms for heat transfer augmentation. Furthermore, a swirl parameter describing swirl flows induced by twisted tape has been developed by balancing the viscous, inertial, and centrifugal forces. The swirl parameter is based on the actual swirl flow velocity next to the tube wall, which is responsible for the shearing stresses. They also proposed a generalized correlation for friction factor in all flow regimes, whereas for the Nusselt number a family of curves, rather than a single expression, describes the transitional behavior of the flow. Nakhchi et al. [8] used double-cut twisted tape to study the heat transfer enhancement of a heat exchanger tube at different cut ratios.

Comparing thermal performance factors with previous works revealed that the new design enhances heat transfer efficiency up to 1.63 for a cut ratio of 0.90. In a recent numerical study [10], the influence of cross-sectional curvature on the performance of twisted tape has been investigated for different twist ratios as well as curve ratios. Although the curvature ratio had no significant impact on pressure drop, the curved twisted tape gives losses on heat transfer efficiency compared with the classically twisted tape.

Although the losses caused by the curvature are not significant at high curve ratios, the effect of low curve ratios becomes significant, especially at higher twist ratios. Along with the geometrical modifications, alterations in the working fluid [11,12] and flow type [13], [14], [15] have also been investigated. Together with the conventional applications of twisted tapes in heat exchangers, some different interesting applications have been reported [16], [17], [18].

Besides the use of various types of twisted tapes, several types of flow turbulators have been incorporated to investigate the heat transfer characteristics in processing applications [19], [20], [21]. Rahmani et al. [22] used motionless spiral inserts to study the temperature blending of two compressible gasses by computational fluid dynamics. They showed that in moderate Reynolds numbers, variable-length inserts significantly improve the temperature blending in comparison to constant-length inserts.

An extensive experimental study was carried out by Garcia et al. [23] to investigate the enhancement of laminar and transitional flow heat transfer using wire coils of different pitches inserted in a smooth tube. The results indicated that pure laminar behavior persists in a wider range of Reynolds numbers in wires with longer pitches. Furthermore, for Reynolds numbers around 1000, the wire inserts increase the heat transfer coefficient up to eight times compared to the plain tube. Keklikcioglu and Ozceyhan [24] experimentally investigated the effect of triangle cross-sectioned coiled-wire inserts on thermo-hydraulic performance in a tube. They concluded that the highest enhancement efficiency could be obtained for the pitch-to-diameter ratio of 1. Moreover, their research effectively examined the influence of triangle position on the destruction of viscous sublayer, which was not studied by previous work [25].

By orienting the triangle vertex to face the flow direction, the flow is distributed into the near wall region, which results in enhanced destruction of the boundary layer. The effect of multiple conical strips on heat transfer behavior as well as flow structures in laminar flow was numerically studied by Liu et al. [26]. The simulations showed that increasing the conical strips in addition to the reduction of pitch results in the augmentation of heat transfer rate and flow resistance. Other techniques for heat transfer enhancement have also been addressed in the literature, including treated and rough surfaces [27], [28], [29] as well as vortex generators [30], [31], [32], [33].

Static or inline mixers are multifunctional motionless devices that are used in chemical reaction and heat transfer applications in addition to mixing, either individually or simultaneously. They can be used for thermal homogenization in continuous tubular reactors [34], or in through-air-drying (TAD) systems to supply a uniform temperature for air flow [35]. Furthermore, cost-effective and environmentally friendly process intensification can be developed by incorporating heat exchangers into the motionless mixers, resulting in high-yield continuous flow reactors.

Several investigations have been implemented to study the performance of inline mixers in heat transfer devices [36], [37], [38], [39], [40]. Simões et al. [38] investigated the effectiveness of Kenics inline mixer for heating supercritical CO₂. Their results revealed that for the same operating conditions, the Kenics mixer gives heat fluxes greater than those for conventional heat exchangers by one order of magnitude. Habchi et al. [41] performed heat transfer simulations of multifunctional heat exchanger-reactors with high-efficiency vortex (HEV) mixer configurations.

The thermal enhancement factor was also used to categorize different arrangements. It is found that adding hemispherical bulges between the arrays of the vortex generator has a great impact on the augmentation of heat transfer while an increase in pressure drop is not significant. In another investigation, Habchi and Azizi [42] examined the influence of a screen-type mixer on mixing and heat transfer. Using screen mixers results in good thermal performance compared to the performance of heat exchangers. Recently, Kwon et al. [40] studied the potential of using additively manufactured inline mixers applied for cooling a heated plate. Their simulation showed that compared to a flat plate, using twisted tape and a chevron mixer with an offset wing considerably improves the heat transfer coefficient.

Simultaneous flow homogenization and heat transfer that are frequently encountered in chemical processing, such as heat exchangers or gas-fed tubular reactors, are essential for the efficient operation of processing units. The use of static mixers, having the potential of both homogenization and heat transfer enhancements, is a promising solution to achieve these goals. Static mixers can efficiently augment heat transfer rates by disrupting the boundary layer near the tube wall and inducing flow rotation. On the other hand, pressure drop challenges in heat transfer enhancement are systematically treated in static mixers as an effective design parameter. These features make static mixers a potential tool for heat transfer enhancement.

Among various types of static mixers, the Blade static mixer has swirl flow characteristics which are very similar to the twisted tapes but with a considerably lower pressure drop. These unique features along with its simple geometry make it an interesting option for use in heat transfer equipment. The blades force the flow outside to the walls, creating a mixing vortex axial to the centerline of the next element and then repeated in opposite directions.

This paper addresses an experimental study to characterize heat transfer enhancement using standard Blade inline mixers as a swirl flow generator. The Blade style element has best-in-class efficient mixing and provides a lower pressure drop compared with other inline mixers. Experiments were carried out in the laminar flow regime and under constant surface heat flux. Correlations were also developed for the friction factor as well as the Nusselt number for the swirl flow induced by the mixer.

Structure of the elements

Since static mixers rely on external pumps to move flow across the mixer elements, pressure drop often serves as a basis for selecting the appropriate mixer. In many cases, the type and number of mixer elements are chosen to affect the best mixing possible without exceeding a maximum allowable pressure drop. The swirl-flow generators used in the experiments belong to a class of Koflo static mixers, called Blade static mixers, which was primarily designed for low

Flow swirling pattern

Heat transfer enhancement induced by swirl-flow generators occurs primarily by the fluid mixing caused by secondary or swirl flow [5]. Flow mixing due to the secondary flow results in higher wall shear stress, which in turn offers a higher friction factor. As mixing intensity increases, the onset of swirl flow takes place earlier and the friction factor increases as the Reynolds number increases. The secondary flow circulation in the tube fitted with Blade insert is typically shown in Fig. 4. At low

Conclusions

An experimental study has been carried out for the characterization of heat transfer enhancement using low-pressure drop static mixers in laminar swirling flow with constant surface heat flux. The tube inserts belong to a class of Koflo mixers called Blade™ static mixers, and are used as swirl-flow generators. The mixer with a 90° crossing angle between the blades provides a higher swirl than the 120° blades, which indicates that the centrifugal force intensifies as the angle between the blades